It depends on what you mean. There are two possible solutions.
1. Reverse the direction of drive of the propeller. This requires a H-bridge driver which the Crazyflie does not have. Would be possible to build on a expansion deck though.
2. Using variable pitch propellers (as on helicopters). I have not seen any design that is small enough for the Crazyflie. It would also require new drive since both the propeller speed and pitch needs to be controlled.

It depends on what you mean. There are two possible solutions.
1. Reverse the direction of drive of the propeller. This requires a H-bridge driver which the Crazyflie does not have. Would be possible to build on a expansion deck though.
2. Using variable pitch propellers (as on helicopters). I have not seen any design that is small enough for the Crazyflie. It would also require new drive since both the propeller speed and pitch needs to be controlled.

I guess H-bridge would be the solution since most of the time, upward thrust is used for normal flight and downward thrust is needed not very often.